There is an increasing emphasis on abatement of volatile organic compounds (VOCs) from industrial emissions and VOC emissions from remedial projects. With the passage of the 1990/1991 Clean Air Amendment, the need for an efficient and cost effective process to control emissions of volatile organic compounds (VOCs) from industrial sources such as paint spray booths, dry cleaning operations, semiconductor manufacture, etc., and from remedial projects such as landfill gas stripping operations, has become increasingly important. Due to the low VOC concentrations typically generated by these sources (&lt;2000 ppmv) processes such as direct incineration and solvent recovery are not economically feasible. However, the concentrations are high enough to shorten the bed life of nonregenerable activated carbon filters, thus making them impractical to use.
Presently the most cost effective process for removing VOCs combines direct thermal oxidation of VOC constiutents and activated carbon adsorption technology. This process which is practiced by the assignee of this application is known as the CADRE process and has been sold and successfully implemented throughout the United States. This process utilizes a single grade or type of adsorbent such as activated carbon, which is contained in two or more cylindrical vessels to adsorb the volatile organic compounds which are introducted to the activated carbon beds in a relatively low concentration gas stream (less than 2000 ppmv). When the activated carbon in a vessel becomes saturated with the VOCs, it is taken off line and the carbon is regenerated by passing a hot gas through the bed countercurrently to the adsorption flow. The hot gas is obtained by taking a portion of the flue gas from a thermal oxidizer used to oxidize the organic compounds and cooling it to the desired regeneration temperature (typically 100.degree.-400.degree. C.).
The thermal oxidizer flue gas is preferably cooled by one of several methods, including diluting the flue gas with ambient air or injecting water into the flue gas stream. The particular method used depends upon a number of factors including the reactivity of the VOCs adsorbed on the carbon bed. For highly reactive VOCs, such as ketones, it is preferable to use water injection to lower the oxygen content of the regeneration gas. Air dilution is preferred, however, where the compounds are relatively nonreactive or where the VOC stream contains chlorinated hydrocarbons which could lead to the formation of corrosive acids if water were present in the regeneration gas.
In the process of regenerating the carbon bed, a small volume of gas containing high concentrations of the VOC's is produced which is then introduced into the thermal oxidizer where greater than 99% of the VOS's are destroyed. Typically, the system is designed such that the onstream adsorption time is much greater than the regeneration time. Thus, the operation of the thermal oxidizer is intermittent, which significantly reduces costs of fuel. In addition, the operation of the process is designed to be continuous in that at least one adsorber is always on line while the other is either being regenerated or on standby. As a result, removal efficiencies of 95% or better are achieved.
Notwithstanding the success and effectiveness of this process, improvements are needed to meet the more stringent VOC emission levels being adopted by environmental and regulatory agencies (at both the state and federal levels) as well as for an ever increasing number of organic compounds.
Accordingly, it is an object of the present invention to provide a method and means for achieving higher removal levels of organic compounds from process gas streams. It is another object of the invention to provide greater efficiencies at higher removal levels for a larger number of organic compounds.